Add code that allows themes to be merged (i.e. if certain elements are
not present in a theme, another theme can be "overlaid" to add missing
elements).
Effectively, this is used to create the logical concept of an "icon
theme", which is loaded after the main theme. So, the main theme can
define colors, and the icon theme fills in any missing elements
(practically, all the icons in the form of prefixes and suffixes).
Icon sets are defined in a theme using the "icons" directive, which
should be an array.
see #17
